1. Field of the Invention
The invention relates to a packet-switched data transmission and more precisely to optimization of data packet numbering, particularly in connection with an acknowledged transmission.
2. Brief Description of Related Developments
In the development of so-called third generation mobile communication systems, for which at least the terms UMTS (Universal Mobile Telecommunication System) and IMT-2000 (International Mobile Telephone System) are used, one starting point has been that they would be so compatible as possible with the second generation mobile communication systems, such as the GSM system (Global System for Mobile Communications). The UMTS core network, for instance, is planned to be implemented on the basis of the GSM core network, and thus the already existing networks can be utilized as efficiently as possible. Further, one aim is to enable the third generation mobile stations to utilize a handover between the UMTS and the GSM. This applies to a packet-switched data transmission as well, particularly between the UMTS and the GPRS (General Packet Radio Service), which is planned to be used in the GSM.
In a packet-switched data transmission, a reliable, i.e. acknowledged, transmission or an unreliable, i.e. unacknowledged, transmission can be used. In the reliable data transmission, the recipient transmits an acknowledgement of the received data packets PDU (Protocol Data Unit) to the transmitter, and the transmitter can transmit the lost or the faulty data packets anew. In the GPRS system, data transmission reliability of the inter-SGSN (Serving GPRS Support Node) handover is secured by means of an 8-bit N-PDU number (Network PDU) associated with the data packets, which helps to check the data packets transmitted to the recipient. In the UMTS system according to the current specifications, reliability of the corresponding handover between serving nodes in a packet-switched data transmission is checked by a 12-bit RLC sequence number of an RLC layer (Radio Link Control) of the packet data protocol.
In a handover between the GPRS and the UMTS, the GPRS is responsible for the reliability of the handover, and so the reliability is arranged to be checked by means of the N-PDU numbers of the GPRS, on the basis of which identification numbers used in the UMTS are created in the handover process. In the handover from the UMTS to the GPRS, the UMTS is responsible for the handover, and the reliability check is based on the identification data of data packets contained in the UMTS. For this purpose, the UMTS system is planned to be provided with an 8-bit data packet number, which is combined as an additional byte with a data packet of the convergence protocol layer PDCP (Packet Data Convergence Protocol) belonging to the UMTS packet data protocol. This PDCP-PDU number thus forms a data packet number logically corresponding to the N-PDU number of the GPRS, and on the basis of this number it is checked in the handover that all data packets have been transferred reliably. It is also possible that the 8-bit PDCP-PDU number is formed of 12-bit RLC sequence numbers by deleting four most significant bits. A corresponding PDCP-PDU numbering, i.e. N-PDU numbering, can also be used in a handover between the UMTS radio network subsystems (so-called SRNS Relocation). Data packets PDU are placed to a buffer to wait until the responsibility for the connection has been transferred to the serving node SGSN of another system or in the intra-UMTS handover to a new serving radio network subsystem SRNS, and the transmitted data packets can be deleted from the buffer each time an acknowledgement of the received data packets is obtained from the recipient.
A problem in the above arrangement is to associate the header field of each data packet of the convergence protocol layer PDCP with the additional byte of the PDCP-PDU number. This increases load in the data transmission, since an additional byte is transmitted in each data packet. The UMTS packet data service does not, however, use the PDCP-PCU number for any purpose in the normal data transmission, but it is only utilized in the handover between the UMTS and the GPRS and in the intra-UMTS handover.
Another problem in the above arrangement is the creation of PDCP-PDU numbers from RLC sequence numbers. RLC numbers are defined sequentially for data units RLC-PDU of the RLC layer. Due to a delay in the system, the buffer may contain a large number of data units RLC-PDU. If the RLC sequence numbers exceed 255, which is the biggest decimal number that can be expressed with eight bits, two or more data packets may have the same PDCP-PDU number, as four most significant bits are deleted from the 12 bits of the RLC sequence numbers. Thus, the recipient can no longer define the data packet to be acknowledged unambiguously on the basis of the PDCP-PDU number of the received data packet, and the reliability of the handover cannot be checked any more.
A further problem may arise in a potential multiplexing of packet data transmissions in the PDCP layer, whereby the RLC layer below the PDCP layer receives data packets from several connections simultaneously. Since the handover reliability is secured on the basis of the bearer connection, defining RLC sequence numbers for many simultaneous connections is very difficult and, in regard to the reliability of the handover, insecure.